Flattened fluid conduits for use in heat exchangers and other systems, and associated methods of manufacture and use

ABSTRACT

Flattened fluid conduits for use in heat exchangers and other systems, and associated methods of manufacture and use are disclosed herein. In one embodiment, the fluid conduit for use in heat exchangers can be manufactured by inserting two or more inner tubes into a circular or at least partially circular barrier tube and at least partially flattening at least some length of the assembly. The heat exchanger conduit can include one or more void spaces within the barrier tube and adjacent to the inner tubes which serve to contain any leaks in the inner tubes and vent any leaks to the atmosphere.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application Ser. No. 61/379,484, filed Sep. 2, 2010, the disclosure of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The following disclosure relates generally to fluid conduits for use in heat exchangers and, more particularly, to dual wall tubing for use in heat exchangers.

BACKGROUND

Copper tubing has many uses in heating, ventilation, and air conditioning (HVAC) applications. Various arrangements of tubing have been used in heat exchangers to hold at least one of the fluids taking part in the heat transfer. These tubing arrangements may leak causing contamination between fluids: if the tubing carrying one of the fluids ruptures the fluids within and without the tube may be mixed.

Dual wall heat exchanger conduits are commonly used in heat pump water heaters to avoid leakages of refrigerant and oils into a potable water supply. Conventional dual wall heat exchanger conduits have utilized a first tube inserted into and pressed against a second tube. Either the first or the second tubes can have protrusions which create a leak path between the tubes. These protrusions can lead to high thermal contact resistance between the tubes, rendering the heat exchanger relatively inefficient.

Some heat exchanger conduits have fluids flowing through the space between the inner tube and the barrier tube. This arrangement poses a risk of fluid cross-contamination in the event of a tube rupture. These heat exchanger conduits further utilize a tapered structure, such that the exchangers are increasingly flattened from end to end in a stepwise or continuous manner to compensate for different fluid properties in warm versus cold regions. This tapering requires more manufacturing complexity than if the exchanger was of a continuous height along all or most of its length.

It would therefore be advantageous to provide a double-walled heat exchanger conduit, having high heat transfer properties, that is relatively easy to manufacture and not prone to detrimental leaks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional end view of a heat exchanger conduit having a circular barrier tube with a plurality of circular inner tubes nested therein, in accordance with an embodiment of the disclosure.

FIG. 1B is a cross-sectional end view of the heat exchanger conduit of FIG. 1A after it has been flattened in accordance with an embodiment of the disclosure.

FIG. 1C is a cross-sectional end view of a flattened heat exchanger conduit configured in accordance with another embodiment of the disclosure.

FIGS. 2A-2C are a series of end views illustrating various cross-sections of the heat exchanger conduit of FIG. 1B at various points along its length.

FIG. 3 is a cross-sectional end view of the heat exchanger conduit of FIG. 1B with a fluid surrounding a barrier tube.

FIG. 4 is an isometric view of a bent heat exchanger conduit configured in accordance with an embodiment of the disclosure.

FIG. 5 is an isometric view of a bent heat exchanger conduit configured in accordance with another embodiment of the disclosure.

FIG. 6 is an isometric view of a bent heat exchanger conduit configured in accordance with another embodiment of the disclosure.

FIG. 7 is a top view of a heat exchanger conduit having alternating lengths of straight flattened sections and rounded bent sections configured in accordance with an embodiment of the disclosure.

FIG. 8 is a cross-sectional end view of a heat exchanger conduit configured in accordance with another embodiment of the disclosure.

FIG. 9 is an isometric view of a heat exchanger having a corrugated outer sleeve configured in accordance with another embodiment of the disclosure.

FIG. 10 is a cross-sectional end view of a flattened heat exchanger conduit having a plurality of grooved inner tubes configured in accordance with another embodiment of the disclosure.

FIG. 11 is a cross-sectional end view of a flattened heat exchanger conduit having a dimpled barrier tube in accordance with a further embodiment of the disclosure.

FIG. 12 is a cross-sectional end view of a flattened heat exchanger conduit having a barrier tube with a plurality of fins configured in accordance with another embodiment of the disclosure.

FIG. 13A is a cross-sectional end view of a heat exchanger conduit having a circular barrier tube with a plurality of pre-flattened inner tubes nested therein, and FIG. 13B is a cross-sectional end view of the heat exchanger conduit of FIG. 13A after the barrier tube has been formed around the inner tubes in accordance with another embodiment of the disclosure.

FIG. 14 is a cross-sectional end view of a heat exchanger conduit having a plurality of inner tubes with inserts positioned therein in accordance with another embodiment of the disclosure.

FIG. 15 is a cross-sectional end view of a heat exchanger conduit having a turbulator and a plurality of inner tubes positioned in an outer sleeve in accordance with yet another embodiment of the disclosure.

DETAILED DESCRIPTION

The present disclosure describes various embodiments of a tubing assembly, such as a heat exchanger conduit, having an outer barrier tube with a plurality of fluid-carrying inner tubes nested therein. In one embodiment, the nested tube heat exchanger conduit can be manufactured by inserting two or more inner tubes into a circular or at least partially circular barrier tube and at least partially flattening at least some length of the assembly. The heat exchanger conduit can include one or more void spaces within the barrier tube and adjacent to the inner tubes which serve to contain any leaks in the inner tubes and vent any leaks to the atmosphere. These and other aspects of the present disclosure are described in greater detail below.

Certain details are set forth in the following description and in FIGS. 1-15 to provide a thorough understanding of various embodiments of the disclosure. Other details describing well-known structures and systems often associated with the manufacturing and use of tubing, flattened tubing, heat exchangers, etc., have not been set forth in the following disclosure to avoid unnecessarily obscuring the description of the various embodiments.

Many of the details, dimensions, angles and other features shown in the Figures are merely illustrative of particular embodiments of the disclosure. Accordingly, other embodiments can have other details, dimensions, angles and features without departing from the spirit or scope of the present invention. In addition, those of ordinary skill in the art will appreciate that further embodiments of the invention can be practiced without several of the details described below.

In the Figures, identical reference numbers identify identical, or at least generally similar, elements. To facilitate the discussion of any particular element, the most significant digit or digits of any reference number refers to the Figure in which that element is first introduced. For example, element 110 is first introduced and discussed with reference to FIG. 1.

FIG. 1A is a cross-sectional end view of a nested multi-tube heat exchanger conduit 100 having an outer barrier tube 110 with a plurality of fluid-carrying inner tubes 112 nested therein. For ease of reference, the outer barrier tube 110 is referred to hereinafter as the “barrier tube 110.” FIG. 1B is a cross-sectional end view of the heat exchanger conduit 100 after it has been flattened in accordance with an embodiment of the disclosure. Referring to FIGS. 1A and 1B together, the inner tubes 112 and barrier tube 110 can be made of various materials known in the art. For example, in certain embodiments, one or more of the tubes 110, 112 can be made of copper, copper alloy, aluminum, stainless steel, etc. In other embodiments, the inner tubes 112 and the barrier tube 110 can be made of different materials, while in further embodiments the inner tubes 112 and barrier tubes 110 can be made of the same material. In yet another embodiment, the inner tube 112 and/or the barrier tube 110 can have a grooved and/or textured surface.

The inner tubes 112 and the barrier tube 110 can have various sizes in accordance with the present disclosure depending on the intended application. In one embodiment, for example, the barrier tube 110 can have an outer diameter D₁ ranging from about 0.3 inch to about 1.4 inches, or from about 0.5 inch to about 0.9 inch, prior to flattening and the inner tubes 112 can have a diameter D₂ ranging from about 0.06 inch to about 0.8 inch, or from about 0.1 inch to about 0.4 inch, prior to flattening. In other embodiments, the inner tubes 112 and/or barrier tube 110 can have diameters D₁, D₂ that fall outside of these ranges. In further embodiments, all or some of the inner tubes 112 may each have different diameters.

When the heat exchanger conduit 100 is flattened, both the barrier tube 110 and the inner tubes 112 take on an oval shape, as illustrated in FIG. 1B. In some embodiments, the ratio between the diameter D₁ of the barrier tube 110 and a flattened height H₁ of the barrier tube 110 ranges from about 2:1 to about 3:1. In some embodiments, the ratio between the diameter D₂ of the individual inner tubes 112 and a flattened height H₂ of the inner tubes 112 ranges from about 1.4:1 to about 2.4:1. In further embodiments, the barrier tube 110 and inner tubes 112 can have ratios between their diameters D₁, D₂ and their flattened heights H₁, H₂ that fall outside of these ranges.

In one embodiment, the inner tubes 112 and the barrier tube 110 can have wall thicknesses T₁, T₂ respectively, ranging from about 0.008 inch to about 0.045 inch, or from about 0.016 inch to about 0.032 inch. In some embodiments, the barrier tube thickness T₁ and the inner tube thickness T₂ can be the same, while in further embodiments the tube thicknesses T₁, T₂ can be different. The length of the heat exchanger conduit 100 can vary in different embodiments of the invention. In some embodiments, for example, the length of the heat exchanger conduit 100 can range from about 1 foot to about 100 feet, or about 3 feet to about 40 feet, depending on the intended use of the heat exchanger conduit 100. For example, for pump-assisted water heater systems, the heat exchanger conduit 100 may be about 10 feet or may be multiple sections of about 3 feet. For systems where the heat exchanger conduit 100 is placed in a tank, the tube may be longer, up to about 40 feet. In yet other embodiments, the heat exchanger conduit 100 can be manufactured in other lengths, depending on the intended application of the conduit 100.

As those of ordinary skill in the art will appreciate, the foregoing dimensions of the nested tube heat exchanger conduit 100 are merely illustrative of various embodiments of heat exchanger conduits configured in accordance with the present disclosure. Accordingly, other embodiments of the present disclosure can include flattened tubes having different diameters, heights, thicknesses, shapes, lengths, etc. depending on the particular application of use and/or a number of different variables including, for example, the wall thickness of the tube, the outer diameter of the tube, the amount of flattening, etc. Therefore, those of ordinary skill in the art will appreciate that various embodiments of the invention described herein are not necessarily limited to any particular tube configuration, but extend to all such configurations falling within the scope of the present disclosure.

In the illustrated heat exchanger conduit 100, three inner tubes 112 are nested within the barrier tube 110. In other embodiments, the heat exchanger conduit 100 can include more or fewer inner tubes 112. The heat exchanger conduit 101 in FIG. 1C, for example, has six inner tubes 112 nested within a barrier tube 111.

During manufacture of the heat exchanger conduit 100, the inner tubes 112 can be placed within the barrier tube 110 using various methods. In one embodiment, for example, the barrier tube 110 can be partially pre-flattened to a height that is between its original diameter D₁ and its final flattened height H₁. Once the barrier tube 110 is partially pre-flattened, the inner tubes 112 can be fed inside the barrier tube 110. Once the inner tubes 112 are nested within the barrier tube 110, the entire heat exchanger conduit 100 can be flattened. In other embodiments, the inner tubes 112 may be fed into the barrier tube 110 prior to flattening the barrier tube. In another embodiment, the barrier tube 110 flattened onto the inner tubes 112 through the use of a suitable “sinking” process. As is known, sinking involves pulling a tube through a die so that the resulting tube has a smaller diameter and the same or increased wall thickness. In further embodiments, the barrier tube 110 may be formed around the inner tubes 112 by stretch reducing or twisting the barrier tube 110 through the application of the Poisson effect. Stretch reducing and/or twisting the barrier tube 110 involves increasing the length of the barrier tube 110 by stretching the barrier tube 110 longitudinally, thereby reducing the diameter of barrier tube 110 until the barrier tube 110 is flattened onto the inner tubes 112.

A number of suitable methods known in the art can be used to flatten the heat exchanger conduit 100. For example, in one embodiment the heat exchanger conduit 100 is compressed to the desired height H₁ using a vice or press. In some embodiments, the heat exchanger conduit 100 is flattened using opposing rollers. The pressure drop of the inner tubes 112 can be measured after flattening to ensure that the tubes 112 remain open along their entire length.

Flattening the heat exchanger conduit 100 increases the surface contact between adjacent inner tubes 112 and between the inner tubes 112 and the barrier tube 110. This increased surface contact lowers the heat transfer resistance between fluids inside the inner tubes 112 and outside the barrier tube 110. Additionally, the flattening operation reduces the hydraulic diameter of the inner tube, which leads to high convective heat transfer rates and reduced refrigerant inventory. In one embodiment, the heat exchanger conduit 100 can be twisted. The twisting aids in forcing the inner tubes 112 and barrier tube 110 together to increase conductive heat transfer among the tubes.

When the heat exchanger conduit 100 is flattened, one or more voids 122 are created adjacent to and between the inner tubes 112. In the illustrated embodiment, the inner tubes 112 contact each other so that the voids 122 are not in fluid communication with each other (i.e. the voids are isolated from each other). The voids 122 may take on various shapes. In one embodiment, for example, at least one of the voids 122 can extend the entire length of the heat exchanger conduit 100. In other embodiments, one or more of the voids 122 can extend only part of the length of the heat exchanger conduit 100. In still further embodiments, the voids 122 can have different shapes along the length of the conduit 100, and/or some portions of the voids 122 may be in fluid communication with each other.

FIGS. 2A-2C are a series of cross-sectional end views illustrating cross-sections of the heat exchanger conduit 100 of FIG. 1B at various points along the length of the heat exchanger conduit 100. In the illustrated embodiment, the tubes 110, 112 are flattened along only a middle portion of the length of the heat exchanger conduit 100. In other words, end portions 240 of the heat exchanger conduit 100 are not flattened but are instead left in a circular configuration. Leaving these end portions 240 circular allows the tubes 112 to be easily joined to a single tube for carrying fluid to other stages in the corresponding heat exchanger. Furthermore, having a circular end portion 240 allows the barrier tube 110 to be connected to other tubes (not shown) by solder or other means using conventional circular end caps or other end fittings. In other embodiments, the heat exchanger conduit can have a flattened, flared, or other cross-sectional shape along its end portions and the heat exchanger conduit 100 can be connected to other tubes using specialized processes or non-circular fittings or end caps.

FIG. 3 is a cross-sectional end view of the heat exchanger conduit 100 with a fluid space 324 surrounding the barrier tube 110. In operation, the heat exchanger conduit 100 transfers heat between a first fluid 330 carried within the ovalized inner carrier tubes 112 and a second fluid 332 carried in the fluid space 324 external to the barrier tube 110. In some embodiments, the fluid space 324 may be contained by an outer sleeve 302 and configured to be a heat exchanger. This flattened tube configuration is highly effective for heat transfer due to the low heat transfer resistance between the fluid transporting tubes 112 and the barrier tube 110. Heat is transferred through the walls of the inner 112 and barrier tubes 110, from the warmer of the first 330 or second 332 fluids to the cooler of the fluids.

The first fluid 330 and second fluid 332 can be any one of numerous fluids. In one embodiment, first fluid 330 can be a working fluid, such as a refrigerant, while second fluid 332 can be potable water. In various embodiments, the first fluid 330 may be any working fluid such as, ammonia, propane, carbon dioxide, steam, or water. In other embodiments, first fluid 330 is a working fluid and second fluid 332 is air. In further embodiments, the first fluid 330 is potable water and the second fluid 332 is a working fluid and/or a gas. In yet further embodiments, the first fluid 330 and the second fluid 332 are the same fluids.

The various embodiments of heat exchanger conduits 100 described herein have several applications for use as heat exchangers. In a heat pump water heater, for instance, water is heated using hot refrigerant gases from a refrigerant compressor discharge. High pressure refrigerant or a refrigerant and oil mixture flows inside the inner tubes 112. As high pressure refrigerant is condensed, it transfers heat to a potable water stream that flows in the fluid space 324. In this embodiment, the barrier tube 110 is made of a material that is compatible with potable water such as copper. The inner tubes 112 must be made of a material and material thickness combination that can withstand the pressure and temperatures of the refrigerant. Making both tubes from copper or a copper alloy is a highly-preferred embodiment, as copper has a high conductivity, is easily formed, and can easily be joined to the other fluid system connections. As pressure is applied from the refrigerant, the higher pressure can cause the inner tubes 112 to have a tendency to round and thus press out on the barrier tube 110. The illustrated configuration has the advantage that when the inner tubes 112 are pressurized, they can press into the barrier tube 110, increasing contact and reducing any resistance to heat transfer. The barrier tube 110 can provide some resistance to this rounding tendency, which can help keep the ovalized shape of the inner tubes 112.

There are numerous other embodiments for using the heat exchanger conduit 100 as a heat exchanger. In a charge air cooler for an engine, for example, the fluid 330 flowing in the inner tubes 112 can be an engine/cooling refrigerant or bottoming cycle working fluid. The fluid 332 in the fluid space 324 outside the barrier tube 110 can be air or an air/exhaust gas mixture. In this embodiment, aluminum or stainless steel may be a preferred material for the inner tubes 112 and/or barrier tube 110. In a heat exchanger using exhaust gas, the fluid 330 flowing inside the inner tubes 112 can be potable water, a bottoming cycle working fluid, or emission reduction fluid. The fluid 332 flowing in the fluid space 324 outside the barrier tube 110 can be exhaust gas. In this embodiment, stainless steel may be a preferred material for the barrier tube 110. In another embodiment, the heat exchanger conduit may be used as a solar water heater with potable water as the outer fluid 332 flowing in fluid space 324 and a heat transfer fluid as the inner fluid 330 flowing in the inner tubes 112. In a further embodiment, the heat exchanger may again be used as a solar water heater, but with potable water as the inner fluid 330 and a heat transfer fluid as the outer fluid 332.

If the inner tubes 112 rupture during operation, the void space 122 serves as a leak containment space and the two heat transfer fluids 330, 332 do not mix. This leak-capturing feature is important in any use of the heat exchanger conduit where it would be problematic or dangerous to have the first 330 and second 332 fluids mix. When heating potable water using refrigerant, for example, having an uncontained rupture in a refrigerant tube could be toxic to the water supply. The same threat arises when potable water is heated using exhaust gases where acids and particulate from the exhaust gas could be toxic to the water. Heat exchange systems on engines or fuel cells also must maintain fluids apart from each other. Otherwise, a contaminated fluid in the power-producing unit could result in toxins in the exhaust or cause the power-producing unit to malfunction.

FIGS. 4-7 are views of heat exchanger conduits having flattened tubes configured in accordance with embodiments of the present disclosure. Referring to FIGS. 4-6 together, in some embodiments of the invention, the heat exchanger conduits 400, 500, 600 can be coiled (i.e. formed into one or more loops), bended, twisted, or otherwise shaped or deformed (hereinafter, collectively referred to as “bended”). Bending the heat exchanger conduits 400, 500, 600 can have advantages in certain applications. First, bending promotes stretching of the metal and can allow for greater contact between the inner 112 and barrier 110 tubes. Second, bending can allow for greater turbulence in the fluid 332 flowing through fluid space 324, outside of the barrier tube 110 and increased heat transfer between fluids 330 and 332. FIG. 7 illustrates an embodiment of the invention having alternating lengths of straight flattened sections and circular bent sections. In each of the alternate embodiments illustrated in FIGS. 4-7, the ability to bend the heat exchanger conduit can allow the design engineer to better use the allowed space and to locate fluid inlet/outlet connections where desired.

FIG. 8 is a cross-sectional end view of a heat exchanger conduit 800 in accordance with an embodiment of the invention. In this embodiment, after nesting inner tubes 812 within a barrier tube 810, the inner tubes 812 can remain in a circular configuration and the barrier tube 810 can be fitted and/or flattened around the inner tubes 812. In this embodiment, as in the embodiments described above, there may be two or more inner tubes 812. In one embodiment, the inner tubes 812 can have individual diameters D₃ and wall thicknesses T₃ that are similar to the diameters D₂ and thickness T₂ described above with reference to the inner tubes 112 in FIG. 1. In other embodiments, the inner tubes 812 can have a greater or lesser wall thickness T₃ and a greater or lesser diameter D₃. In one embodiment, the inner tubes 812 can have a diameter D₃ of about 0.375 inch. In yet another embodiment the inner 812 and/or barrier 810 tubes can have a grooved or other texture.

As with the embodiments discussed above, one or more voids 822 are created adjacent to and between the inner tubes 812 when the heat exchanger conduit 800 is flattened. In some embodiments, the inner tubes 812 contact each other and the voids 822 are not in fluid communication with each other. The voids 822 may take on various shapes. In one embodiment, for example, at least one of the voids 822 can extend the entire length of the heat exchanger conduit 800. In other embodiments, one or more voids 822 can extend only part of the length of the heat exchanger conduit 800. In still further embodiments, the voids 822 can have different shapes along the length of the heat exchanger conduit 800.

FIG. 9 is an isometric view of a heat exchanger 900 having a barrier tube 910 and a plurality of inner tubes 912 flattened and formed into a coil in accordance with another embodiment of the present disclosure. In this embodiment, a corrugated outer sleeve 902 having a circular cross section is fitted over the barrier tube 910. In other embodiments, the cross section of outer sleeve 902 may have other shapes such as, for example, an oval. The corrugated outer sleeve 902 can be made from a suitable material for the application and may also be dimpled or flattened. In other embodiments, the outer sleeve 902 may be made from other materials suitable for the transport of, for example, potable water.

End fittings 940 and 942 are attached to opposite ends of the outer sleeve 902, fluidly sealing the outer sleeve 902 to the barrier tube 910. The end fittings 940 and 942 each have a fluid inlet and/or outlet. In the illustrated embodiment, a first fluid (e.g. a working fluid such as a refrigerant) flows through the inner tubes 912. A second fluid (e.g. a fluid to be cooled or heated by the working fluid) can enter the heat exchanger 900 through the inlet of end fittings 940, flow through the fluid space between the barrier tube 910 and the outer sleeve 902, and exit at the outlet of the end fitting 942 having been cooled or heated by the first fluid.

Forming the heat exchanger 900 into a coil can induce secondary flows in the first fluid flowing through the inner tubes 912 and in the second fluid flowing through the fluid space between the barrier tube 910 and the outer sleeve 902. Secondary flows can occur when a fluid flows through a coiled tube and can be caused by a pressure gradient between the inner wall and outer wall of the coiled tube. Secondary flows can cause increased turbulence in the first and second fluid flows, thereby increasing heat transfer between the two fluids.

FIGS. 10-12 are cross-sectional end views of heat exchanger conduits 1000, 1100 & 1200, respectively, having flattened tubes configured in accordance with embodiments of the present disclosure. Referring to FIGS. 10-12 together, in some embodiments the heat exchanger conduits 1000, 1100, and 1200 can be configured to increase fluid turbulence in a plurality of inner tubes 1012 and/or in a fluid space 1024, external to a barrier tube 1010, thereby increasing heat transfer between a first fluid 1030 flowing through the inner tubes 1012 and a second fluid 1032 flowing through the fluid space 1024. In some embodiments, the fluid space 1024 may be enclosed by an outer sleeve 1002. FIG. 10 illustrates one embodiment in which grooves 1014 are formed on an interior surface portion of the inner tubes 1012. In other embodiments, the grooves 1014 may be formed on an exterior surface portion of barrier tube 1010 and/or on an interior surface portion of the sleeve 1002. FIG. 11 illustrates another embodiment in which dimples 1116 are created in a sidewall of barrier tube 1010 that at least partially protrude into a sidewall of one or more of the inner tubes 1012. Creating one or more dimples can allow for increased surface contact between the inner tubes 1012 and the barrier tube 1010, thereby facilitating greater heat transfer between the first fluid 1030 and the second fluid 1032. FIG. 12 illustrates a further embodiment in which one or more fins 1218 are formed on an exterior surface of barrier tube 1010. The fins 1218 can increase turbulence of the outer fluid 1032, again increasing heat transfer between the working fluid 1030 and the outer fluid 1030. The fins may be formed using various suitable methods known in the art, such as welding, bonding, skiving, etc.

FIG. 13A is a cross-sectional end view of a heat exchanger conduit 1300 having a barrier tube 1310 (e.g. a circular barrier tube) with a plurality of pre-flattened inner tubes 1312 nested therein, and FIG. 13B is a cross-sectional end view of a heat exchanger 1301 after the barrier tube 1310 is formed around the inner tubes 1312. The barrier tube 1310 can be formed around the inner tubes 1312 using various suitable methods known in the art, such as rolling, shrinking, twisting, etc. In the illustrated embodiment, a working fluid 1330 flows through the inner tubes 1312 and an outer fluid 1332 flows through a fluid space 1324 external to the barrier tube 1310. In some embodiments, the fluid space 1324 may be contained by an outer sleeve 1302. The resulting conduit, in a straight, coiled, or twisted configuration, exposes increased external surface area to the outer fluid 1332 flowing through the fluid space 1324 and can also cause increased turbulence in the outer fluid 1332, thereby allowing increased heat transfer between inner fluid 1330 and outer fluid 1332.

FIG. 14 is a cross-sectional end view of a heat exchanger conduit 1400 having a barrier tube 1410 and a plurality of flattened inner tubes 1412 nested therein, in accordance with another embodiment of the present disclosure. In the illustrated embodiment, an insert 1413 is disposed in each of the inner tubes 1412, and each insert 1413 includes a cavity 1414 sealed from the inner fluid 1430. In other embodiments, the inserts may be a solid rod, a bar, and/or a corrugated strip. The use of the inserts 1413 in the interiors of the inner tubes 1412 decreases the hydraulic diameters of the inner tubes 1412, thereby allowing increased heat transfer between a first fluid 1430 and a second fluid 1432 flowing through a fluid space 1424. The insert 1413 also serves as an impediment to further flattening of the inner tubes 1412, such that when pressure is applied to the barrier tube 1410 there is increased thermal contact between the barrier tube 1410 and inner tubes 1412. As discussed above, in some embodiments, the fluid space 1424 may be contained within an outer sleeve 1402.

FIG. 15 is a cross-sectional end view of a heat exchanger conduit 1500 having a plurality of flattened tubes 1512, a corresponding barrier tube 1510, and two turbulators 1506 in an outer sleeve 1502, in accordance with one embodiment of the present disclosure. In this embodiment, a first fluid 1530 flows through the inner tubes 1512 and a second fluid 1532 flows through a fluid space 1524. The turbulators 1506 are located in the fluid space 1524. The turbulators 1506 can be springs, twisted tape, star beads, or other turbulators known in the art. The turbulators 1506 in the fluid space 1524 can induce increased turbulence in the outer fluid 1532, thereby allowing increased heat transfer between the inner fluid 1530 and the outer fluid 1532.

From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the various embodiments of the invention. Further, while various advantages associated with certain embodiments of the invention have been described above in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the invention. Accordingly, the invention is not limited, except as by the appended claims. 

I/we claim:
 1. A heat exchanger conduit comprising: a barrier tube; and a plurality of fluid-carrying inner tubes nested within the barrier tube, wherein each individual inner tube is in contact with at least one other individual inner tube to create a plurality of isolated voids between the inner tubes and the barrier tube.
 2. The heat exchanger conduit of claim 1 wherein the individual inner tubes are at least partially flattened.
 3. The heat exchanger conduit of claim 1 wherein the individual inner tubes have an oval cross-sectional shape.
 4. The heat exchanger conduit of claim 1 wherein the barrier tube and the plurality of inner tubes are at least partially flattened.
 5. The heat exchanger conduit of claim 1 wherein at least one of the individual inner tubes has a different diameter than at least one of the other individual inner tubes.
 6. The heat exchanger conduit of claim 1 wherein at least one of the barrier tube and the plurality of inner tubes has an end portion with a generally circular cross-sectional shape configured to sealably join to an adjacent conduit for fluid communication therebetween.
 7. The heat exchanger conduit of claim 1 wherein the barrier tube and the plurality of fluid-carrying inner tubes are formed into a coil comprising one or more loops.
 8. The heat exchanger conduit of claim 1 wherein at least one of the inner tubes includes a plurality of grooves on an interior surface portion thereof.
 9. The heat exchanger conduit of claim 1 wherein a sidewall portion of the barrier tube protrudes into a sidewall portion of at least one of the inner tubes.
 10. The heat exchanger conduit of claim 1, further comprising an insert disposed in at least one of the inner tubes.
 11. The heat exchanger conduit of claim 1 wherein at least one of the outer barrier tube and fluid-carrying inner tubes is copper.
 12. A heat exchanger comprising: an outer sleeve; a flattened barrier tube disposed within the outer sleeve to define a fluid space therebetween; and a plurality of flattened inner tubes nested within the barrier tube, wherein each individual inner tube is in contact with the barrier tube and at least one adjacent inner tube.
 13. The heat exchanger of claim 12 wherein the outer sleeve is corrugated.
 14. The heat exchanger of claim 12 wherein a sidewall portion of the flattened barrier tube protrudes into a sidewall portion of at least one of the flattened inner tubes.
 15. The heat exchanger of claim 12 wherein the barrier tube includes a plurality of fins extending outwardly into the fluid space.
 16. The heat exchanger of claim 12 wherein the fluid space generally surrounds the barrier tube, and wherein the heat exchanger further comprises: a first fluid flowing through at least one of the inner tubes; and a second fluid, different than the first fluid, flowing through the fluid space.
 17. The heat exchanger of claim 12, further comprising at least one turbulator disposed in the fluid space between the barrier tube and the outer sleeve.
 18. A method of manufacturing a heat exchanger conduit comprising: disposing a plurality of first tubes within a second tube; and flattening at least a portion of the second tube whereby each individual tube of the plurality of first tubes contacts the second tube and at least one other individual first tube to create at least one isolated void between the plurality of first tubes and the second tube.
 19. The method of claim 18, further comprising flattening the plurality of first tubes within the second tube.
 20. The method of claim 18, further comprising disposing the second tube into a sleeve to define an open fluid space therebetween.
 21. The method of claim 18, further comprising forming the second tube into a coil comprising one or more loops.
 22. The method of claim 18, further comprising forming grooves on an interior surface portion of at least one of the first tubes.
 23. The method of claim 18, further comprising forming a plurality of fins on an exterior surface portion of the second tube.
 24. The method of claim 18, further comprising twisting at least a portion of the second tube and the plurality of first tubes nested therein. 